This invention pertains to a disposable filter and method for removing microaggregates from blood in limited transfusion of infants. Transfusion of premature infants and very young children present special problems in the removal of microaggregates in the blood and blood components that have been stored. The extremely small vein and capillary size of premature and neonatal infants creates hazards to damage by introduction of microembulisms or microaggregates into the circulatory system. Limited transfusions of 5 to 20 cc of blood create a desirability for a small, disposable filter which may be utilized in conjunction with an infusion needle or infusion set with stopcock and syringe.
The need for filtration of blood has been known for many, many years. In situations where a typical 18 gauge needle was utilized for getting blood into patients, such needles might become plugged with clots when the blood coming from blood banks was not treated in any manner. This problem led to the development of the 170 micron clot screen, which despite advances in handling of blood by blood banks, is still used today in some instances and is often included as part of a blood administration set.
In addition to this problem from the past, advancing medical technology has made manifest the need for finer filtration so as to protect the capillary bed of the pulmonary vasculature, thereby avoiding the possibility of the Adult Respiratory Distress Syndrome, Shock Lung or other Functional Pulmonary Abnormalities. Changes in blood are manifested almost immediately upon withdrawal from the body and are enhanced in every contact made with a foreign surface which will tend to create microaggregates. Even the type of anticoagulant used in the container for storage may influence the rate of microaggregate formation. As set forth above, microaggregates begin to form almost immediately and by the third day after removal from a human body, there are marked increases in the formation of microaggregates, and within one week the aggregation process is generally complete. Microaggregates are typically composed of platelets, nonviable leukocytes, fibrin strands (fibrinoproteins), denatured proteins and portions of cellular membranes. Microaggregates can be found in all types of blood, blood components and fractionates, i.e. whole blood, packed cells, fresh frozen plasma, cryoprecipitates and AHF concentrates. Applicant's filter will permit infusion of viable blood components such as erythrocytes, leukocytes, platelets, Factor VIII and Factor IX for treatment of hemophiliacs and other blood disorders. The volume of the microaggregates increase daily as the blood is stored.
The size and number of microaggregates is estimated to be 7.times.10.sup.7 particles that are 10 microns or larger in one unit of seven-day old blood. Slightly more than one-half of the microaggregates are smaller than 20 microns in diameter. With each unit of blood, 10 to 15 million microaggregates in the range of 20 to 40 microns in diameter can be carried into the capillary bed of the pulmonary vasculature. It has also been learned that the terminal precapillary arterioles are estimated to number 2.70.times.10.sup.8 and are 20 microns or less in diameter. From the above information, the importance of microaggregate filtration to remove debris in the form of microaggregates from blood and blood products has become clear.
In dealing with pediatric age group patients, the problems of blood filtration become even more critical since the pulmonary vasculature and capillary network of pediatric patients is smaller in size than in adults. The usage of microaggregate blood filters is a common practice in adult patients. However, the only filters presently available on the market are for the adult population. An example of this is shown in U.S. Pat. No. 3,701,433 issued to S. Krakaur et al. Application of adult blood filters to the administration of blood to children and neonates can cause several problems. First, the cost of an adult blood filter is relatively high, particularly when considering the frequent need for small volume infusions of filtered blood in the neonate age group. Secondly, large priming volumes are associated with adult blood filters which may exceed, by as much as 10 to 20 percent of the neonate total blood volumn. By way of example, as much as 70 ml. of blood can be wasted in priming certain adult blood filters. Thirdly, adult blood filters must be used "in-line" associated with a standard blood administration set, whereas, for small volume infusion of neonates and children, blood and blood components are generally administered by syringe. A syringe is used in order to achieve a degree of accuracy which is needed when dealing with the critical balances demanded by the neonate in particular. The above reasons, and perhaps others, effectively exclude filtration of blood, blood components and blood fractionates in the neonate group. There has long existed in the art a need for a microaggregate-type filter for pediatric patients and a method which will provide filtration of blood, blood components and other blood products to remove undesired microaggregates, while still allowing the viable components to be infused in the patient. At the same time the blood, blood components and other blood products must be treated in a gentle manner so as not to damage them.
With the discovery of the existence of microaggregates in blood stored prior to use, numerous approaches to filtration were developed. Massive transfusion involving the utilization of tens or dozens of units of blood create special problems in filtration, and solutions have been developed. The limited or special problems occur in small transfusions of premature babies and small infants. Generally, a small, compact, disposable filter is desirable in these types of transfusions.
Among the numerous types of blood filters for the filtration of microemboli which have been developed are:
The Intercept filter (the gross clot screen filter) employs an initial screen filter of approximately 170 microns. The depth filter is of woven Dacron and a final screen filter with a pore size of 20 microns.
One of the more popular and widely used filters is the Pall filter which was primarily developed for utilization in cardiopulmonary bypass in open heart surgery. This filter has a relatively long life, high capacity, and employs a clot screen and a folted, woven polyester screen with resulting 40-micron passageways.
The Biotest microfilter employs nylon screen mesh decreasing in aperture size from 200 microns down to a final state of 10 microns.
Depth filters generally employ a mass of fibrous elements through which the blood passes filtering by absorption.
The Bentley filter utilizes a type of polyurethane foam of graduated size.
The Fenwal blood filter utilizes a combination of polyester foam and more restricted portion is constructed of packed nylon fibers.
The Swank filter is an improved or modification of the old glass wool filter and utilizes Dacron wool as the filter medium.
Prior art filters frequently require from 15 to 80 cc to prime the filter. The device of this invention requires essentially no priming in that the volumetric capacity of the filter is less than 1 cc. The design of this invention results in substantial savings in blood and blood components in transfusions of infants and pediatric patients.
An object of this invention is to provide a new and useful method and apparatus for microfiltration of blood in infants which overcomes the problems associated with the prior art. Applicant's invention provides an inexpensive and effective apparatus and method for transfusion of infants. At the same time, it eliminates any of the problems associated with the prior art, which, so far as known, did not provide a solution to the problem. Other objects of the invention will become apparent from the remainder of the specification.